Method and apparatus for detecting fluid influx in offshore drilling operations

ABSTRACT

An improved method and apparatus for use in connection with offshore drilling is disclosed. The system is particularly useful in operations where a drilling vessel is situated at the surface of a body of water and circulation of drilling fluid is accomplished by introducing drilling fluid into a drill string extending from the vessel into a borehole in the floor of the body of water and returning it through a separate conduit. The present invention includes method and apparatus for terminating circulation of the drilling fluid and then monitoring the fluid level of the drill fluid within the return conduit to detect any influx of formation fluid into the borehole.

R seaoaeno United States Patent 1 [111 3,809,170 llfrey et a1. May 7, 1974 METHOD AND APPARATUS FOR 3,384,178 5/1968 Agnew et a1 175/25 X DETECTING FLUID INFLUX IN OFFSHORE 3,595,075 7/1971 Dower DRHLLING OPERATIONS 3,613,806 10/ 1971 Malott 3,614,761 10/1971 Rehm et a1. 73/155 X [75] Inventors: William T. Ilfrey; James R.

Sheffield both of Houston Primary ExaminerWerner H. Schroeder [73] Assignee: Esso Production Research Company, Attorney, Agent; Firm-"James Gilchrist Houston, Tex.

[22] Filed: Mar. 13, 1972 [57] ABSTRACT [21] Appl. No; 234,226 An improved method and apparatus for use in connection with offshore drilling is disclosed. The system is particularly useful in operations where a drilling vessel [52] US. Cl 175/7, 166/.5, ll755/4685, i Situated at the Surface ofa body of water and Circw Im Cl E2 1b Z 4 lation of drilling fluid is accomplished by introducing Field of Search 175, drilling fluid into a (11111 string extending from the vessel into a borehole in the floor of the body of water and returning it through a separate conduit. The present invention includes method and apparatus for ter- [56] References C'ted minating circulation of the drilling fluid and then mon- UNITED STATES PATENTS itoring the fluid level of the drill fluid within the return 3,612,176 10/1971 Bauer 166/.5 conduit to detect any influx of formation fluid into the 2,964,116 12/1960 Peterson..... 175/48 borehole,

3,268,017 8/1966 Yarbrough 175/25 3,324,7-17 6/1967 Brooks et al 175/48 X 9 Claims. 1 Drawing Figure I1 ll I U U 1 METHOD AND APPARATUS FOR DETECTING FLUID INFLUX IN OFFSHORE DRILLING OPERATIONS BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention pertains to early detection of the entry of formation fluids .into the borehole during drilling operations conducted offshore from floating vessels.

2. Description of the Prior Art Detection and control of the entry of formation fluids into the borehole is an important aspect of any drilling safety program. Early detection of these fluid kicks is particularly critical in offshore drilling. operations where the wellhead and blowout preventers are positioned on the floor of the body of water. Because of theirdistance from the drill rig and their principle of operation, the subsea preventers have a reaction time, i.e., time required to close in the well and restore control, that may be substantially greater than that required onshore. Early detection is additionally important because the drilling riser, an extension of the borehole from wellhead to vessel, is a large diameter conduit which will not withstand well pressure. Thus, control of pressurized fluids must be accomplished well before the fluids reach the equipment positioned at the water surface.

One method used for kick detection onshore is to periodically measure the volume of fluid in the mud pits to detect any change between the rate of flux of drill fluid into and out of the borehole. If mud is being returned to the pits more rapidly than it is being pumped down the hole, it is normally indicative of the influx of formation fluids. Unfortunately, accurate gauging of the mud pits during floating drilling operations is precluded by vessel motion except in calm water. Other onshore detection techniques rely on pressure fluctuations of the drilling fluid in the standpipe. These are also obscured on a floating rig by pressure surges caused by vessel heave opening and closing the telescoping joint positioned within the drill string for motion compensation. There therefore exists a need for a system which will permit early detection of formation fluid entry and which will not be adversely affected by vessel motion.

SUMMARY OF THE INVENTION The present invention alleviates the problems outlined above and permits early detection of fluid influx in wells drilled from floating drilling vessels. The present invention involves terminating circulation of drilfrom the vessel into a borehole in the floor of the body of water and routing it back to the vessel through a return conduit extending between the vessel and a subsea wellhead. In accordance with the method of the invention, circulation of the drilling fluid is terminated and thereafter the fluid level of the drilling fluid within the return conduit is logged for a short period of time to detect increases in fluid level characteristic of fluid influx. The apparatus of the invention includes a means for generating a signal whenever circulation of drilling fluid is terminated and a means for withdrawing drilling fluid from the return conduit in response to this signal to lower the level of the fluid therein. A means is also provided for monitoring the fluid level of the drilling fluid within the return conduit. This preferably comprises a pressure sensor situated on the return conduit, and, in a preferred embodiment, includes at least two vertically spaced pressure transducers.

The return conduit will normally have a discharge line situated near its upper end through which drilling fluid is free to flow to the mud pits. Since the fluid level is normally at the discharge line level, influx will normally result in drilling fluid flowing to the mud pits instead of raising the fluid level, and hence pressure, within the return conduit. In accordance with the present invention the fluid level is lowered from the discharge line level and then observed for a short time period, preferably by monitoring hydrostatic pressure, to detect changes characteristic of fluid influx. During the making of a connection or at such other times during the course of drilling at which it is desired to test for influx, it is desirable to lower the fluid level within the return conduit by withdrawing a quantity of fluid from it. During a trip out of the hole the withdrawing of sections of the drill string from the borehole will normally lower the fluid level within the conductor pipe sufficiently to render fluid withdrawals unnecessary.

The present invention permits early detection of the entry of formationfluidsduring the course of offshore drilling operations so that corrective action can be promptly initiated. Themethod and apparatus of the present invention will therefore be seen to offer significant advantages over systems existing heretofore.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic elevation view, partially in section, of a drilling vessel floating on a body of water and provided with apparatus embodying the present invention.

DESCRIPTIONOF THE PREFERRED EMBODIMENTS The drawing depicts one embodiment of apparatus suitable for use in carrying out the method of the invention. Shown in FIG. 1 is a drilling vessel 11 situated on a body of water 13. The vessel will be understood to include the normal complement of equipment used in offshore drilling operations although only part of this apparatus is shown. The portion of the drilling fluid circulation system depicted includes the upper end of the drill string 15 through which the mud is introduced into the hole under pressure. The drill string is shown extending downwardly into bell nipple 17 which is attached to the upper end of riser string 25. The bell nipple serves to guide the drill string into the upper end of the slip joint and includes a discharge conduit 21 which diverts the returns to the mud storage pits, not shown, which are also situated aboard the drilling vessel. Normally both the end of the discharge line and the upper end of the bell nipple are open to the atmosphere. The upper slip joint barrel 19 telescopes within the outer slip joint barrel 23 positioned below it to protect the portion of the riser extending downwardly from the lower end against stresses induced by vessel heave. Tensioning lines 27 lead from padeyes on the outer barrel to a tensioning system aboard the vessel which serves to support the riser string and maintain substantially constant tension in it.

Shown positioned at separate points along the outer barrel of the slip joint, although they could be positioned anywhere along the return conduit, are two pressure transducers designated by 31; Electrical cables 33 are depicted as a preferred means for transmitting pressure responsive signals from two vertically spaced points on the riser string to a control means on the vessel designated by numeral 35. Positioned above the transducers, and preferably along the inner barrel of the slip joint at least 8-l0 feet below the mud discharge nae; is an auxiliary mud discharge Tine 371m ing a valve 39 to control flow. Electrical cable 41 is exemplary of one way the position of the valve could be regulated and hence flow of fluid through the auxiliary discharge line could be governed by the control means situated on the vessel.

While the means for monitoring the fluid level within the return conduit depicted in the drawing is responsive to pressure, and this is the preferred means, numerous variations consistent with the present invention will be readily apparent. For example, the fluid level could be monitored acoustically or by resistivity determinations at a series of vertically spaced points within the return conduit. Changes in fluid level within the return conduit can normally be inferred with sufficient accuracy from changes in the hydrostatic pressure of the drilling fluid; however, preferably two vertically spaced pres sure sensors will be employed to permit determination of both hydrostatic pressure and pressure gradient. From this information the elevation of the liquid level in the return conduit can be accurately determined, as by means of an analog computer, and any increase in elevation when circulation is stopped can be observed so that influx of formation fluids can readily be ascertained.

During normal drilling operations the level of the drilling fluid within the return conduit is at the discharge line. Any fluid influx following a halt in circulation will therefore merely flow through the discharge line to the mud pits without creating any change in pressure in the riser. One means to assure that any influx creates an increase in fluid level with an attendant increase in pressure is to close off the discharge line so that the liquid level in the riser string will build up if there is any influx. The system shown in FIG. 1 is also very useful in this connection. In operation, upon a drop in drill fluid pressure, as for example is caused by termination of circulation preparatory to the making of a connection, control means 35 automatically opens the valve on the auxiliary discharge line to drain the fluid contained in the inner barrel between the auxiliary and normal discharge lines. The valve closes as soon as the fluid is withdrawn and the liquid level thus lowered. Pressure is then monitored to detect any increase in fluid level.

In some situations heave of the vessel and resultant displacement of the slip joint are great enough that the pressure behavior of the column of drilling fluid can termination of circulation the fluid level in the return line will normally be reduced to a level corresponding to the greatest contraction of the telescoping joint as a result of flow through the discharge line. Absent influx, the fluid level should thus remain constant and any increase in liquid level, although of a transitory nature, will be indicative of entry of fluid into the borehole.

A particularly critical time for fluid kicks is during trips of the pipe string to change the bit. If pipe is withdrawn from the hole too rapidly, the resultant swabbing action may promote entry of formation fluid. It is thus particularly desirable to watch the fluid level in the return conduit as the pipe is coming out ofthe hole. Unfortunately, this is rendered somewhat more difficult by fluctuations in fluid level caused by the successive reductions of fluid level each time a new stand of pipe is withdrawn followed by the successive increases in fluid level as the riser is refilled to prevent any substantial loss of hydrostatic head. It may therefore be desirable to halt the refilling operation from time to time long enough to check for fluid influx. Alternatively, the pattern of peaks and troughs in the fluid level may be plotsatisfactorily be monitored without withdrawing fluid from the return conduit. At the crest of each wave the slip joint is extended, whereas when the vessel is in the trough this telescoping joint will be contracted. After ted against time and examined for irregularities suggestive of fluid influx. For example, if refilling is continued at a constant rate, any entry of formation fluid into the wellbore will accelerate the observed rate of fluid level increase.

What is claimed is:

1. In a method of drilling from a vessel floating on the surface of a body of water wherein circulation of drilling fluid is accomplished by introducing drilling fluid into a drill string extending downward from said vessel into a borehole in the floor of said body of water, routing upward through a return conduit and discharging it back to the vessel, the improvement comprising terminating circulation of the drilling fluid, preventing further discharge of tluid from said return conduit and logging the fluid level of said drilling fluid within said conduit to detect any changes therein.

2. The method of claim 1 wherein discharge of fluid from the return conduit is prevented by lowering the fluid level of said drilling fluid within said return conduit following termination of circulation.

3. The method of claim 2 wherein said fluid level is lowered by withdrawing a portion of said drill string from said borehole.

4. The method of claim 2 wherein said fluid level is lowered by withdrawing a quantity of drilling fluid from said return conduit.

5. The method of claim 1 wherein said fluid level is logged by logging the hydrostatic pressure of the fluid within said return conduit.

6. The method of claim 5 wherein said hydrostatic pressure is logged at at least two vertically spaced points on said return conduit.

7. In apparatus for offshore drilling of the type wherein a drilling vessel is situated at the surface of the body of water, a drill string extends from said vessel through a wellhead situated on the floor of the body of water and into a borehole extending therebelow, a return conduit extends from said wellhead to said vessel and drilling fluid is circulated down said drill string and returned to said vessel through said conduit, the improvement which comprises:

a. means on said vessel for generating a signal in response to a termination in circulation of said drilling fluid;

monitoring said fluid levelfincludes a pressure sensor situated on said return conduit.

9. The apparatus of claim 8 wherein said pressure sensor includes at leasttwo pressure transducers spaced vertically on said return conduit. 

1. In a method of drilling from a vessel floating on the surface of a body of water wherein circulation of drilling fluid is accomplished by introducing drilling fluid into a drill string extending downward from said vessel into a borehole in the floor of said body of water, routing upward through a return conduit and discharging it back to the vessel, the improvement comprising terminating circulation of the drilling fluid, preventing further discharge of fluid from said return conduit and logging the fluid level of said drilling fluid within said conduit to detect any changes therein.
 2. The method of claim 1 wherein discharge of fluid from the return conduit is prevented by lowering the fluid level of said drilling fluid within said return conduit following termination of circulation.
 3. The method of claim 2 wherein said fluid level is lowered by withdrawing a portion of said drill string from said borehole.
 4. The method of claim 2 wherein said fluid level is lowered by withdrawing a quantity of drilling fluid from said return conduit.
 5. The method of claim 1 wherein said fluid level is logged by logging the hydrostatic pressure of the fluid within said return conduit.
 6. The method of claim 5 wherein said hydrostatic pressure is logged at at least two vertically spaced points on said return conduit.
 7. In apparatus for offshore drilling of the type wherein a drilling vessel is situated at the surface of the body of water, a drill string extends from said vessel through a wellhead situated on the floor of the body of water and into a borehole extending therebelow, a return conduit extends from said wellhead to said vessel and drilling fluid is circulated down said drill string and returned to said vessel through said conduit, the improvement which comprises: a. means on said vessel for generating a signal in response to a termination in circulation of said drilling fluid; b. means for withdrawing drilling fluid from said return conduit in response to said signal to lower the level of the fluid contained therein; and c. means for monitoring the fluid level of the drilling fluid within said return conduit.
 8. The apparatus of claim 7 wherein said means for monitoring said fluid level includes a pressure sensor situated on said return conduit.
 9. The apparatus of claim 8 wherein said pressure sensor includes at least two pressure transducers spaced vertically on said return conduit. 